1. Field of the Invention
The present invention relates generally to an optical distance measuring apparatus operable in accordance with a phase difference detecting process. More particularly, the present invention relates to an optical distance measuring apparatus of the aforementioned type wherein not only a distance between the apparatus and a stationary object but also a distance between the apparatus and a moving object can reliably be measured based on a phase difference between the electrical phase of a modulated signal associated with a distance measuring light beam to be emitted from a light source and the electrical phase of a demodulated signal derived from a reflected light beam reflected from the object. Further, the present invention relates to an optical distance measuring apparatus of the aforementioned type wherein a speed of a moving object can be measured with reference to the measured distance between the apparatus and the moving object.
2. Description of the Prior Art
To facilitate understanding of the present invention, a typical conventional optical distance measuring apparatus of the aforementioned type will briefly be described below with reference to FIG. 2. It should be noted that illustration of an optical system of the apparatus is neglected for the purpose of simplification, because the optical system employable for the optical distance measuring apparatus is well known to those skilled in the art and it is believed that a particular detailed description of the optical system is not required.
Referring to FIG. 2, the apparatus includes an optical intensity modulating circuit 1 and a reference oscillator 2. A reference signal outputted from the reference oscillator 2 is modulated in the optical intensity modulating circuit 1 so as to allow a modulated distance measuring light beam L-1 to be emitted toward an object to be measured (not shown) from a light source (not shown) electrically connected to the optical intensity modulating circuit 2. Subsequently, a reflected light beam L-2 reflected from the object to be measured is received by a light receiving element (not shown) which in turn activates a demodulating circuit 6 and a filter 7. The demodulated signal derived from the reflected light beam L-2, the modulated signal modulated in the light intensity modulating circuit 1 and the locally oscillated signal outputted from a local oscillator 3 are inputted into mixers 4 and 5, respectively, in which these signals are subjected to frequency conversion.
Thereafter, a phase difference between the electrical phase of the modulated signal and the electrical phase of the demodulated signal is detected and calculated with reference to the electrical phase of the locally oscillated signal with the aid of a phase meter 8 so as to determine a distance between the apparatus and the object.
The conventional optical distance measuring apparatus constructed in the above-described manner is basically constructed in order to measure a distance between the apparatus and a certain stationary object with a high accuracy. For this reason, there does not arise any particular problems even though several seconds are taken until a single distance measuring operation is completed.
In practice, a plurality of modulated signals are used for performing a distance measuring operation for measuring a distance of order of several kilometers between the apparatus and the object with an accuracy represented by a millimeter as a unit. To assure that data representing the distance therebetween are reliable in respect of accuracy, a number of distance measuring operations are successively performed and the results derived from the successive distance measuring operations are averaged so as to obtain an average value. Then, this average value is practically used as a reliable value representing the distance between the apparatus and the object to be measured with a high accuracy.
However, in a case where a distance between the apparatus and a moving object is to be measured, there arises a problem that a measuring operation should be completed in a shorter period of time rather than with a high accuracy.
In view of the above-noted problem, when a distance between the apparatus and a moving object is measured with the conventional optical distance measuring apparatus as mentioned above, it is necessary that a number of measuring operations are successively performed while using the corresponding number of frequencies, causing a long time to be taken until a single datum representing the measured distance is obtained. For this reason, it is not satisfactorily acceptable to employ the conventional optical distance measuring apparatus when a distance between the apparatus and a moving object is to be measured.